Other Heteroatom-Centered Radicals

Various other heteroatom-centered radicals have been generated as initiating species. These include silicon-, sulfur-, selenium- (see 3.4.3.1). nitrogen- and phosphorus-centered species (see 3.4.3.2). Kinetic data for reactions of these radicals with monomers is summarized in Table 3.10. 

Silicon centered radicals can be generated by transfer to silanes and by photolysis of polysilanes. Rate constants for addition to monomer are several orders of magnitude higher than similar carbon centered radicals.453,43 The radicals have nucleophilic character. 

Tabic 3.10 Selected Rate Data for Reactions of Heteroatom-Centered Radicals 

Reports of the intramolecular addition of heteroatomic radicals other than alkoxyl, aminyl, and thiyl are rather scarce, although very interesting and unexpected behavior has often been reported. 

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In Tab. 15.2 the relative reactivity of seleno radicals toward unsaturated bonds is compared with that of other heteroatom-centered radicals [132, 136]. 

Sulfur- and other heteroatom-centered radical cations from organic compounds are the subject of the third section. The fact that such species could exhibit relatively long lifetimes of up to milliseconds was an initially somewhat surprising but most welcome property for their investigation. The present survey summarizes some of their optical and redox properties, and reveals parameters controlling their decay. 

SULFUR- AND OTHER HETEROATOM-CENTERED RADICAL CATIONS... 

Other radical reactions not covered in this chapter are mentioned in the chapters that follow. These include additions to systems other than carbon-carbon double bonds [e.g. additions to aromatic systems (Section 3.4.2.2.1) and strained ring systems (Section 4.4.2)], transfer of heteroatoms [eg. chain transfer to disulfides (Section 6.2.2.2) and halocarbons (Section 6.2.2.4)] or groups of atoms [eg. in RAFT polymerization (Section 9.5.3)], and radical-radical reactions involving heteroatom-centered radicals or metal complexes [e g. in inhibition (Sections 3.5.2 and 5.3), NMP (Section 9.3.6) and ATRP (Section 9.4)]. 

The hydrogen abstraction addition ratio is generally greater in reactions of heteroatom-centered radicals than it is with carbon-centered radicals. One factor is the relative strengths of the bonds being formed and broken in the two reactions (Table 1.6). The difference in exothermicity (A) between abstraction and addition reactions is much greater for heteroatom-centered radicals than it is for carbon-centered radicals. For example, for an alkoxy as opposed to an alkyl radical, abstraction is favored over addition by ca 30 kJ mol"1. The extent to which this is reflected in the rates of addition and abstraction will, however, depend on the particular substrate and the other influences discussed above. 

From a pulse radiolysis study on the S04 -induced reactions of Thd (Deeble et al. 1990), it has been concluded that the pKa of the Thd radical cation (deprotonation at N(3)) should be near 3.5, i.e. close to that at N( 1) in Thy. It is noted that also in the parent, Thy, the pKa values at N( ) and at N(3) are quite close. A Fourier-transform EPR study using photoexcited anthraquinone-2,5-disulfonic acid to oxidize Cyt and IMeCyt shows that the radical cation of the former de-protonates rapidly at N(l) while that of the latter deprotonates at the exocylic amino group (Geimer et al. 2000). The EPR evidence for the formation of heteroatom-centered radicals by S04 in its reactions with some other pyrimidines (Bansal and Fessenden 1978 Hildenbrand et al. 1989 Catterall et al. 1992) is in agreement with a marked acidity of such radical cations. It is re-emphasized that this conclusion does not require that radical cations are formed in the primary step. 

The availability of radical clocks that are a-substituted carbon-centered radicals or heteroatom-centered radicals is limited, however. Several experimental difficulties have limited progress in measurements of absolute rate constants for these types of radicals. One problem is the lack of precision for low-temperature ESR studies, and another has been a limited number of reactions available for production of radicals in LFP studies. A third fundamental problem affects the types of LFP studies described above for Bu3SnH specifically, the UV absorbance of the tin-centered radical is weak, and its signal can be obscured by absorbances of other species. 

All of what has been described for sulfur-centered radicals and ions applies, in principle, to corresponding species with other heteroatoms as radical site. Whether it is three- or five-electron bonds, the basis of their establishment is always the interaction of a singly occupied orbital with a lone pair orbital and promotion of one of the electrons into the lowest lying antibonding orbital. The following gives a brief survey on examples with heteroatoms other than sulfur. 

Radicals Centered on Other Heteroatoms, Organic Radical Ions... 

The immediate consequence of adopting this electronic structure is an elongation of the sulfur-sulfur bond which, in turn, facilitates its rupture. On the other hand, the 2c/3e bond exhibits a sufficient degree of stability over the separated thiyl and thiolate constituents. In conclusion, this electronic ala arrangement provides the rationale for the establishment of equilibrium (14) and the entire RS7(RS SR) relationship. Incidentally, such odd-electron bonds are quite a common type of bond in heteroatom-centered radicals and radical ions and many more examples will be discussed and a more detailed electronic picture will be given later in connection with the oxidation of organic sulfides (Section 3). 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

The volume is divided grossly into sections dealing with individual types of free radicals such as carbon-centered radicals, nitrogen-centered radicals, nitroxyl radicals, oxygen-centered radicals and radicals, centered on other heteroatoms. These sections deal mainly with irreversible reactions. In addition, there are sections on reversible electron and proton transfer processes and their equilibria and a chapter on biradicals. An index of radicals formulae will facilitate data retrieval. 

Finally, it must be said that while the main features concerning the cyclization of unsaturated 0-, N-, or S-centered radicals are beginning to be understood, very little is known about the behavior of other heteroatomic radicals, although some very interesting features emerge from the first reports published. 

Only one heterocyclic cycloproparene having the heteroatom in the ring adjacent to the cyclopropene has been isolated, namely the l,l-dimethylcyclopropa[c]pyri-dine derivative 170 which is available in moderate yield upon photochemical extrusion of Nj from the 3Ff-indazole 169. In contrast, irradiation of 171 produces no cycloproparene 173, but other products derived from the intermediate biradical 172. Apparently, the radical centers in 172 are too far apart to allow ring-closure. ... 

Structures. The methyl radical is planar and has D symmetry. Probably all other carbon-centerd free radicals with alkyl or heteroatom substituents are best described as shallow pyramids, driven by the necessity to stabilize the SOMO by hybridization or to align the SOMO for more efficient pi-type overlap with adjacent bonds. The planarity of the methyl radical has been attributed to steric repulsion between the H atoms [138]. The C center may be treated as planar for the purpose of constructing orbital interaction diagrams. 

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